Acoustic coupling in telephone device handsets is common and occurs when signals broadcast by the speaker of a telephone device are coupled to the microphone of the same telephone device. This acoustic coupling causes echo signals in the network over which the communication channel is established. The acoustic suppression of signals broadcast by the handset speaker before they are picked up by the handset microphone is variable and depends on a number of factors, such as for example, the pressure exerted on the user's ear by the handset speaker and how well the handset speaker seals the user's ear. The loss in power of these signals as a result of acoustic suppression can vary from about −50 db to as low as −24 db when the telephone device handset is placed on a hard surface. If delays in the network are short (i.e. 30 ms), the normal acoustic suppression of signals broadcast by the handset speaker is generally sufficient to inhibit voice quality from being noticeably degraded by echo signals picked up by the handset microphone. However, if the delays in the network are significant, echo signals in the network due to acoustic coupling will noticeably degrade voice quality over the communication channel. As such, suppressing echo signals resulting from acoustic coupling in telephone devices is important.
Echo cancellers in telephone devices have been considered. A typical echo canceller attempts to model the transfer function of the echo signal path using a linear algorithm such as a Least-Mean-Squared (LMS) algorithm. The estimated echo signals generated by the echo canceller are subtracted from the echo signals picked up by the handset microphone. Differences between the estimated echo signals and the actual echo signals result in error signals, which are fed back to the echo canceller. Unfortunately, since the algorithm executed by the echo canceller is linear, the echo canceller cannot deal with non-linear effects and can only converge to a transfer function that approximates the echo signals. As a result, residual echo signals propagate through the echo canceller to the network.
To deal with the deficiencies noted above, an echo suppressor has been considered and is described in U.K. Patent Application Serial No. 9907102.9 filed on Mar. 26, 1999. This echo suppressor includes a power level calculator that determines the power level of signals broadcast by the handset speaker. A mask generator is responsive to the power level calculator and generates masks that are a function of the determined power levels to suppress echo signals received by the handset microphone. To avoid noticeable switching effects in the presence of background noise, the echo suppressor can be conditioned to generate leaky masks. This is achieved by setting one or a few of the least significant bits (LSBs) in the masks to one (1) so that the lower bits of the received echo signals are leaked through the echo suppressor.
Although this echo suppressor works well to suppress echo, since the number of least significant bits in the masks that are set to one to leak echo signals is static, switching effects that degrade voice quality still result. Also, since the echo suppressor is active all the time with the number of zeros in the masks changing depending on the power level of the signals broadcast by the handset speaker, the echo suppressor degrades speech quality during double-talk situations. As will be appreciated, improvements in echo cancellation/suppression are desired.
It is therefore an object of the present invention to provide a novel echo suppressor and method for suppressing echoes in a communication path.